The present invention concerns a method for dewatering a paper web which is conducted through a multiroller press assembly comprising at least three consecutive press nips defined by rollls interacting with each other and through which nips the continuous paper web coming from the wire passes, being continually supported between the nips.
The present invention further relates generally to an improved method in a paper machine press section for dewatering a paper web efficiently yet gently so that the paper produced obtains advantageous structural and strength properties. In this connection particular attention is directed to the pressing procedure in the first press nip wherein the run of the press felts has been arranged and the press rolls selected with a view towards achieving a very high effectiveness in the dewatering while simultaneously providing a symmetrical dewatering and thus also a symmetrical sheet structure.
It is well known that in conventional pressing methods, as much water as possible is attempted to be removed from the paper web mechanically, whereupon the paper web is conducted into the drying section of the machine, where the rest of the water is removed from the paper web by evaporation. The most important and most serious problem in dewatering a web by pressing it between press rolls is that if one tries to remove water too efficiently and too rapidly, the fibre network in the paper web is not preserved well enough intact. In that case the natural strength of the web suffers, which has a negative effect both on the running of the paper machine and on the quality and other characteristics of the completed paper. In conventional paper machine press sections, the web is carried by a felt and is conducted into a pressing nip formed by two rolls. In such a nip the dewatering takes place due to the pressure applied to the web between these rolls. Conventionally, one roll has a plain surface facing the web whereas the other roll which is typically wrapped by said press felt usually has a recessed surface of some kind. In earlier low speed paper machines, two such dewatering press nips are provided, each nip being formed by a pair of rolls as described above.
The amount of water removed by such pressing is proportional to the pressure applied and also to the dwell time of the web in the press nip, i.e., to the time during which the web is under pressure. In high-speed paper machines where the web is conducted rapidly through the press nips, the pressing time becomes relatively short. To make the total time during which the web is subjected to the pressing treatment sufficiently long to achieve adequate dewatering, there must be several nips in the press section. Of course, the broader the nip is, the longer is the pressing time. A thick and soft press felt used between press rolls broadens the nip width.
In pressing methods of conventional press assemblies of the prior art, it also easily happens that, owing to conventional dewatering methods, the structural symmetry of the paper web cannot be preserved, which has a detrimental effect on the quality of the produced paper. The dewatering taking place in the first press nip of the press assembly has a decisive effect as regards the characteristics of the paper that is manufactured. Prior to entry into the press treatment, the paper web contains water in an amount equal to about 4 to 6 times the amount of fibres found in it. However, the water content depends to a great extent on the speed of the paper machine. Thus, at very high speeds, the amount of water in the web leaving the wire section may be 9 to 10 times the fibre content. In the first press nip, the water is removed relatively easily even by a relatively slight pressure. However, special arrangements as regards press roll types and press felts are required. When the web, containing a relatively great amount of water, is pressed, part of the fibres in the web tend to move along with the escaping water, thus suffering a displacement. The original fibre and material distribution in the paper web may even substantially change if dewatering in the press nip takes place in one direction only, through one face of the paper web. This is in fact what occurs in conventional press nips, where one surface of the paper web is opposed by a smooth roll surface and the other surface is towards a felt, under which there is e.g., a foraminous or groove roll surface.